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Pros

Pro

Easily export your pen

CodePen makes it really easy to export code as a zip or Github Gist.

Pro

Real time output

Pro

Lots of support for frameworks and preprocessors

CodePen has an impressive amount of support for preprocessors (such as Jade, Haml, Slim, Sass, Less, Stylus, Coffeescript and PostCSS). There is also plenty of frameworks and libraries to pick from (Foundation, Bootstrap, Angular, D3, Backbone, Ember etc.).

Pro

Great display/profile page

The codepen profile page allows you to display all of your public pens, and control which order you want them to be viewed in. This is great for showing off your work to possible employers, other devs etc..

Pro

Easily fork pens

To fork a pen only requires clicking one button, and you'll be able to modify the pen on your own account.

Pro

Great community pens

You can search through other pens, either by keyword or popular, picked or recent.

Cons

Con

No private pens with free account

There is an option on Codepen for private pens, however it requires upgrading to a Pro account ($9/month).